1. Field of the Invention
This invention relates to a foamed ceramic body comprising at least 90% by weight of a ceramic composed of SiO.sub.2, Al.sub.2 O.sub.3, an alkali metal oxide and CaO, and to a process for its production. The foamed ceramic body is suitable as a building material such as an exterior wall panel.
2. Description of the Prior Art
It is well known that foamed ceramic bodies have properties suitable for use as a building material, for example light weight, excellent thermal insulation and high durability. No technique, however, has yet been developed which can industrially produce foamed ceramic bodies of practical utility.
For the foamed ceramic bodies to have utility as fire-retardant or refractory building materials, they must meet certain requirements. Firstly, they must endure temperatures of 800.degree. C., preferably 900.degree. C. the fire preventing and resisting tests stipulated in JIS A1302-1975 and JIS A1304-1975. Secondly, they are required to be produced as a large-sized plate for practical purposes. Desirably, such a large-sized plate of ceramic foams is produced by heat-foaming on a plate or wire, or in a mold, of a heat-resistant metal such as stainless steel. In view of the heat resistance of the metallic material, the heat-foaming temperature should be not more than 1100.degree. C., preferably not more than 1000.degree. C. Thirdly, for commercial practice, they should be produced continuously. To achieve this, it is desirable that cracks and the like should not be caused by a difference in temperature between the surface portion and the inside portion of the heat-foamed plate even when its cooling is rapid.
The present inventors have reviewed and studied conventional foamed ceramic bodies, but have failed to find any which satisfies all of the above requirements.
For example, a formed inorganic body of a multilayer structure prepared from volcanic glass using varying amounts of a blowing agent (Japanese Laid-Open Patent Publication No. 22010/1977) and an inorganic material composed of a mixture of a slag and a silicic acid-containing substance (Japanese Laid-OPen Patent Publication No. 65716/1979) require heat-foaming temperatures of 1100.degree. to 1300.degree. C. which are outside the range mentioned above with regard to the second requirement.
An inorganic material composed of a mixture of "shirasu" which occurs in Kagoshima Prefecture of Japan, a slag and water glass (Japanese Laid-Open Patent Publication No. 35730/1978) contains a large amount of water glass so that its heat-foaming temperature can be lowered to about 1000.degree. C. However, since sodium exudes to near the surface of the material, pores in the surface portion become large, and pores in the inside are small. It is impossible therefore to obtain a uniformly foamed product.